1. Field of the Invention
The present invention relates to a photosensor, and more particularly to a color photosensor for reading a color image.
2. Description of the Prior Art
Conventionally, in a photoelectric conversion apparatus used as an optical input unit of an image data processing apparatus such as a facsimile device, a digital reproduction device or a character reader, it is well known to use a photosensor as a photoelectric conversion element. Especially, a strip line sensor which includes a one-dimensional array of photosensor is recently used to constitute part of a highly sensitive image processing apparatus.
As an example of one of the photosensors which constitute a strip line sensor, a planar photoelectric photosensor could be named which includes a photosensitive layer which may be made of charcogenide, CdS, CdSSe, amorphous silicon (referred to as a-Si hereinafter) or the like, and a pair of opposing metal electrodes disposed on the layer to form a gap which functions as a photoreceptor.
Planar photoelectric photosensors each comprising a-Si, especially the a-Si which includes hydrogen atoms and/or halogen atoms (fluorine, chlorine or the like), referred to as a-Si(H,X) hereinafter, have high optical response speeds and provide large output photocurrents, thereby constituting an excellent one-dimensional photosensor array. The a-Si(H,X) materials are unpollutant and very productive because silicon technology such as plasma CVD technique or photolithopatterning technique can be adopted.
In addition, as shown in FIG. 19, a-Si(H,X) photosensors are characterized in that their spectral sensitivity is close to spectral luminous efficiency so that they are very suitable for color photosensors. More particularly, the spectral sensitivity of the a-Si(H,X) photosensors is flat. That is when the photosensors are irradiated with 450 nm blue light, 550 nm green light, and 650 nm red light, each having the same energy of 10 .mu.W/cm.sup.2, they provide output photocurrents 2, 3.5 and 3, respectively. Thus they are very suitable for color photosensors.
In order to provide such a color photosensor, a color filter and a photosensor can be separately formed and then stuck together. Alternatively, a color filter can be provided directly on a photoelectric section. In the former method, respective color elements of the filter and photoelectric conversion elements of the photosensor are required to be registered precisely with each other, so that it is difficult to apply this method to a strip line sensor or the like. On the other hand, in the latter method, if a color filter is provided directly on a photoconductive conversion section, no precise register is required to stick the respective color elements of the color filter and the photoelectric conversion section of the photosensor. This method can be effected in the same photoprocess and also applied satisfactorily to the construction of a strip line sensor or the like.
A color filter used in such a color photosensor must have enough durability to light and heat due to incident light.
The color filter is formed in part of the process of forming the photosensor. Thus, the filter forming process is required which does not influence the characteristics of the a-Si photoconductive layer. An additional requisite for the color filter is that a highly accurate filter pattern can be formed on a rugged photosensor surface.
As a conventional color filter, a dyeing color filter is known which includes mordanting layer made of a hydrophilic macromolecule material which may include gelatin, casein, glue or polyvinyl alcohol, the mordanting layer being dyed with coloring matter. In this dyeing method, many dyestuffs are usable and accommodation to the spectral characteristic required of the filter is relatively easy. The dyeing step includes a hard-to-control step of dipping the mordanting layer into a dyeing bath in which a dyestuff is dissolved. In addition, a complicated step of providing an intermediate resist printing layer for each color is included, thereby resulting in a low yield. The heat resistance of the filter materials is relatively low, i.e., 150.degree.-160.degree. C., so that the filter materials cannot be used in the steps in which heat treatment is required.
A dyeing process including a wet treatment adversely affects the characteristics of amorphous silicon and is unrecommendable. Furthermore, it is difficult to form a highly accurate pattern on a rugged surface.
In contrast, an evaporation method is known which forms a coloring matter film of a dyestuff or pigment using a gas phase evaporation method which includes a vapor deposit step, etc. (Japanese Unexamined Patent Publication (Kokai) No. 146406/1980 etc.). This method includes the steps of evaporating a color material into a film, forming a mask with a resist and forming a color element by the wet etching which includes patterning the color material, which constitutes the film, with a solvent which selectively dissolves the color material, or the dry etching which includes ashing the color material with gaseous plasma ions. These steps are repeated for a required number of color filter color elements through corresponding transparent protective films, which protect photosensors and already formed color elements from being etched. In the wet etching method, it is difficult to select a solvent to dissolve only the color material without affecting the resist mask. It is also difficult to prevent etching of the color material which underlies the resist mask due to utilization of dissolution of the color material, thereby rendering it difficult to obtain minute shapes. These drawbacks are not found in the dry etching method.
However, when the evaporation film is patterned by dry etching, the film as well as the resist mask are removed simultaneously. Thus, the resist mask must be considerably thick, thereby degrading the accuracy of the resulting pattern. One transparent intermediate protective film is needed each time a single color element is formed so as not to damage the photosensor itself and color elements of the color filter already patterned by the dry etching. The presence of this protective film and the resist mask decreases the transmission factor of the filter, thereby increasing flare light energy. In addition, although the color material film itself has an excellent thermal resistance because it is formed by evaporation, the whole color filter has a degraded thermal resistance because the intermediate protective film and resist mask have degraded thermal resistance.
An a-Si(H,X) forming method which forms the photoconductive layer of the sensor unit of a color photosensor such as described above includes plasma CVD method, reactive sputtering method, or ion plating method. Each of these methods uses a glow discharge to expedite the reaction. However, in order to obtain an excellent a-Si(H,X) film having a high photoconductivity using any one of the methods, the film must be formed with relatively low discharge power. However, the photoconductive layer obtained by formation of the film with such low discharge power is not fully adhered to a substrate made of a glass or ceramic material, and the film is likely to be peeled off when the photoconductive layer is subjected to a subsequent photolithography step, etc., for formation of the electrodes.
In order to prevent the film from being peeled off, a method is conventionally employed which includes roughening a substrate surface and thereafter depositing an a-Si substance on the roughened substrate surface. That is, the substrate surface is in advance etched chemically with hydrofluoric acid, for example, or otherwide scratched physically with a brush, for example. However, such method has the following drawbacks:
(1) A system associated with the washing line has a complicated structure and is expensive when a chemical such as hydrofluoride is used; PA1 (2) It is difficult to control the degree of a ruggedness on the substrate surface; PA1 (3) Microscopic defects are likely to occur at the substrate surface when this surface is roughened.
The characteristics of an a-Si film deposited on the microscopic defects and hence the characteristics of the resulting photoconductive layer are likely to vary from place to place. Therefore, when a color photosensor is formed with photosensor units having such a-Si(H,X) films, since the characteristics of the sensor units differ from each other, a correction circuit must be additionally provided which accommodates a discrepancy between the characteristics of the photosensor units in order to obtain an appropriate color signal.